A long term evolution advanced (LTE-Advanced) network standard has been developed to provide wireless data rates of 1 Gbps downlink and 500 Mbps uplink. The LTE-Advanced network standard also offers multi-carrier transmission and reception within a single band as well as multi-carrier transmission and reception within two separate bands. Multi-carrier transmission within a single band is referred to as intra-band transmission and reception. In contrast, multi-carrier transmission and reception within two different bands is referred to as inter-band transmission and reception. LTE-Advanced technology is also known as fourth generation (4G) technology.
LTE-Advanced operation requires a simultaneous dual carrier transmission in the same band (i.e., intra-band) and into different bands (i.e., inter-band). A transmission of dual LTE-Advanced carriers in a single band in a non-contiguous manner will result in an increased peak-to average ratio (PAR) of around 1 dB. This increase is over an increase of about 1 dB of PAR due to a use of clustered single carrier frequency division multiple access (SC-FDMA). The combined increase in PAR results in a significant negative impact on efficiency of a transmitter chain made up of a transceiver and a power amplifier (PA).
In this regard, envelope following techniques for linear modulation are highly desirable for LTE-Advanced customers and others in the years to come because envelope following and pseudo-envelope following enables a very efficient use of energy. Envelope following techniques employ envelope following systems that are power management systems that control power amplifiers (PAs) in such a way that the PA collector/drain voltage (referred to herein as VCC) follows an RF input signal envelope. The RF input signal envelope is an instantaneous voltage of a PA input RF signal, (referred to herein as VIN).
Implementing pseudo-envelope following improves overall efficiency of PA systems because a power management function is realized using high efficiency switcher systems. However, using envelope following techniques is not practical for transmitter chains that involve dual intra-band carriers due to a large bandwidth requirement that would be placed on a typical switching power supply. The reason for the large bandwidth requirement is that bandwidth is a function of frequency separation between the dual intra-band carriers. For the purpose of this disclosure, envelope following systems include pseudo-envelope following systems, wherein pseudo-envelope following is envelope tracking that includes power amplifier (PA) collector/drain voltage pre-distortion to ameliorate power amplifier nonlinearity. It should be understood that envelope following is sometimes referred to as envelope tracking by some.
Lack of practical envelope following systems present a major challenge for realizing front end radio architectures (FERAs) that are necessary for providing multi-carrier operation using intra-band and inter-band transmission and reception. FERAs that do not employ envelope following systems cannot operate efficiently due to the extra 2 dB of PAR.
FIG. 1 is a schematic of a related art front end radio architecture (FERA) 10 that is not configured to accept power from power management architectures that employ envelope following. The FERA 10 includes a transmitter block 12 for transmitting LTE Advanced multi-carrier signals. The FERA 10 also includes a first power amplifier (PA) 14 powered by a first switcher 16 and a second PA 18 powered by a second switcher 20.
A first duplexer 22 for an RF band A and a first receive (RX) diversity/multiple-input multiple-output (MIMO) filter 24 for an RF band B are coupled between the first PA 14 and a first band switch 26. The first duplexer 22 and the first RX diversity/MIMO filter 24 are selectively coupled to a first antenna 28 through the first band switch 26. The first duplexer 22 outputs signals RX_A captured by the first antenna 28. The first RX diversity/MIMO filter 24 outputs signals RX_B_DIV also captured by the first antenna 28. The band switch 26 is controlled by a control signal CTRL1.
A second duplexer 30 for the RF band B and a second RX diversity/MIMO filter 32 are coupled between the second PA 18 and a second band switch 34. The second duplexer 30 is selectively coupled to a second antenna 36 through the second band switch 34. The second duplexer 30 outputs signals RX_B captured by the second antenna 36. The second RX diversity/MIMO filter 32 outputs signals RX_A_DIV also captured by the second antenna 36.
The transmitter block 12 includes a first transmitter 38, a first RF modulator 40, a first radio frequency (RF) phase locked loop (PLL) 42, a second transmitter 44, a second RF modulator 46, and a second RF PLL 48. The transmitter block 12 further includes a multi-carrier combiner 50 for combining signals output from the first RF modulator 40 and the second RF modulator 46.
The related art FERA 10 can operate in an intra-band multi-carrier mode. During operation of the related art FERA 10 in the intra-band multi-carrier mode, the first transmitter 38 outputs analog baseband (ABB) signals to the first RF modulator 40. Similarly, the second transmitter 44 outputs ABB signals to the second RF modulator 46. In response, the first RF modulator 40 in cooperation with the first RF PLL 42 outputs a first carrier within the RF band A while the second RF modulator 46 in cooperation with the second RF PLL 48 outputs a second carrier that is also within the band A. The first PA 14 provides power amplification of the first carrier and the second carrier which are output through the first duplexer 22 to the first antenna 28.
The related art FERA 10 also includes an inter-band multicarrier mode. During operation of the related art FERA 10 using the inter-band multi-carrier mode, the first RF modulator 40 in cooperation with the first RF PLL 42 outputs a first carrier within the RF band A while the second RF modulator 46 in cooperation with the second RF PLL 48 outputs a second carrier within the RF band B. The first PA 14 provides power amplification of the first carrier which is output through the first duplexer 22 to the first antenna 28. The second PA 18 provides power amplification of the second carrier which is output through the second duplexer 30 to the second antenna 36.
While the related art FERA 10 offers a realizable architecture for LTE-Advanced operation, the related art FERA 10 is wasteful with regard to energy, in that the FERA 10 is not structured to take advantage of a high energy efficiency operation provided by envelope following systems. Energy efficiency in battery powered user equipment (UE) such as mobile terminals that implement LTE-Advanced operation is very important, since a relatively long operation time between battery charges is desirable.
FIG. 2 is a spectrum diagram that illustrates a common collector voltage (VCC) bandwidth (BW) switcher modulation requirement for intra-band dual carrier transmission. In particular, the modulation bandwidth of the switcher 16 (FIG. 1) and the switcher 20 (FIG. 1) is a function of an offset frequency Df between a carrier #1 and a carrier #2. Therefore, the higher the offset frequency Df between the carrier #1 and the carrier #2, the higher the modulation bandwidth must be. At some point, the offset frequency Df is large enough that related art approaches for modulating a VCC pseudo envelope following (PEF) signal via either the switcher 16 or the switcher 20 are no longer practical. For example, if the offset frequency is 40 MHz, then the supply modulation bandwidth needed for envelope tracking is about 1.5×(40 MHz+20 MHz) or about 90 MHz for LTE-Advanced carriers having around 20 MHz of bandwidth. A multiplier of 1.5 is a result of a square root operation of PEF calculation. Moreover, even if the offset frequency Df is equal to zero between two adjacent carriers having a 20 MHz bandwidth each, a resulting 50 MHz VCC BW is too large for efficient modulation of the VCC PEF via either the switcher 16 or the switcher 20. Thus, there is a need to practically meet the VCC BW switcher modulation bandwidth requirement in order to implement LTE-Advanced operation in a more efficient manner than is possible with the related art FERA 10.